The present invention relates to a method for purifying acetic acid by removing therefrom by-products such as formaldehyde by-produced in the production of acetic acid.
As for the method for producing acetic acid, various methods have been employed, such as the one which relies on oxidation of acetaldehyde, another one which relies on oxidation of hydrocarbons such as naphtha, butane or the like, a further one which relies on a synthesis from methanol and carbon monoxide, etc. In any of these methods, however, since various kinds of by-products are formed during the process of oxidation, etc., it is necessary to remove them. As for such by-products, aldehydes, formic acid, methyl alcohol, methyl acetate, etc. are mentioned although the kinds vary depending upon the methods for producing acetic acid. These by-products can be usually removed by purification through distillation, but it is difficult to remove reducible substances such as formic acid, formaldehyde, etc. by means of a mere distillation. For removing such by-products to obtain purified acetic acid, various methods have been also proposed such as treatment with an oxidizing agent e.g. potassium permanganate, etc., chemical treatment e.g. hydrogenation treatment, etc. followed by distillation, removal through azeotropic distillation with chloroform, benzene, etc. However, even in case of technical grade of acetic acid, it has become a recent tendency that acetic acid containing only an extremely small amount of impurities is required. Particularly for acetic acid for fermentation use, for example those employed in producing sodium glutamate, lysine, ribonucleic acid, etc., acetic acid which is used as a solvent in producing terephthalic acid, etc., it has now become necessary to determine the content of formaldehyde by way of a standard test for products, in addition to the past potassium permanganate test for determining reducible substances, and in this case, 30 ppm or less has become now a required value of the content. The prior art, however, has been directed only to removal or separation of reducible substances, particularly formic acid, and there is almost none which relates to removal of formaldehyde. Only Japanese Pat. No. 267113 discloses a method for separating formaldehyde by an azeotropic distillation in the presence of an auxiliary substance such as isopropyl acetate or the like. However, the content of formaldehyde in the acetic acid obtained by the purification according to this method is as high as 300-500 ppm. Such a value does not satisfy the above-mentioned requirement.